This invention relates generally to electric power system arresters, and more specifically to communications interface accessories for electric power system arresters.
An arrester is a protective device utilized as part of electric power transmission and distribution, which is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert over-voltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When exposed to an over-voltage condition, the arrester operates in a low-impedance mode that provides a current path to electrical ground having a relatively low impedance. The arrester otherwise operates in a high-impedance mode that provides a current path to ground having a relatively high impedance. The impedance of the current path is substantially lower than the impedance of the equipment being protected by the arrester when the arrester is operating in the low-impedance mode, and is otherwise substantially higher than the impedance of the protected equipment. Upon completion of the over-voltage condition, the arrester returns to operation in the high-impedance mode. This prevents normal current at the system frequency from following the surge current to ground along the current path through the arrester.
To remove a short circuit of line potential conductors connected to the arrester upon an arrester failure, a disconnect device is provided between the arrester and electrical ground. This type of disconnect device is sometimes referred to as a disconnector, or an isolator, and such devices are commercially available from Cooper Power Systems of Waukesha, Wis. Known disconnectors may include an internal resistor connected in parallel with a spark gap assembly, and a black powder charge in an unprimed .22 caliber cartridge that is heat activated. Thus, if the arrester was to fail and a sustained current was to flow through the disconnector, a spark would be generated by the spark gap assembly. Heat from the spark would detonate the powder charge and mechanically sever an electrical connection between the arrester and electrical ground. Short circuit conditions through the arrester may therefore be removed.
Electrical power transmission and distribution equipment typically is configured to deliver electricity from a power station or generator to customers. However, electric power systems have also been used to transmit and receive communications signals. For example, power lines have been used by utility companies to transmit and receive communications signals to monitor equipment and to read meters. Power lines have also been used to provide broadband communications for customers. These communications may be referred to as broadband over power line (BPL) communications. Various techniques have been developed to couple broadband communications signals to medium-voltage power lines. These broadband communications signals typically occupy frequencies in the 2-50 MHz region. One approach to coupling communications signals to these medium-voltage power lines is to use the intrinsic capacitance of metal oxide varistor (MOV) arresters to couple a portion of the communications radio frequency signals onto medium-voltage power lines.